1. Field of the Invention
The present invention is directed to a method and apparatus for continuously, directly monitoring the RF-tightness of a shielded cubicle in an MR system of the type wherein a radio-frequency signal is transmitted onto the cubicle from the outside via an antenna and the signal part that passes through to the cubicle interior is identified with a receiver in the inside of the cubicle
2. Description of the Prior Art
RF shielded cubicles and their faultless functioning are responsible in part for the image quality that can be achieved with MR systems. A leaky RF cubicle can lead to image disturbances as a result of discrete noise sources and to a reduction of the signal-to-noise ratio that is achieved. A critical and highly stressed element is the RF seal of the access doors. Despite a properly closed door, this can slowly or even suddenly deteriorate due to dirty or bent seal springs without being directly detected.
Conventionally, this problem has been addressed by means of regular shield attenuation measurements were implemented in regular system maintenance (by a service technician). Monitoring contacts that detect whether the door is mechanically closed are a further known measure. This monitoring of the closed door, however, is of no use whatsoever when the RF-tightness deteriorates due to contamination or the aforementioned bending of the seal springs. Regular maintenance, of course, ensues only at intervals of weeks or even months, so that an image disruption due to leaks of the radio-frequency seal and of the access door of a shielded cubicle can be present over a longer time span and significantly degrade the MR exposures during this time span.
For monitoring the RF tightness of all types of cubicles, IEEE Std 299-1997, 21 Apr. 1998 describes a mechanism wherein a radio-frequency signal is transmitted onto the cubicle from the outside via an antenna and the signal part that passes to the interior is identified with a receiver in the inside of the cubicle. The measurement arrangement that is disclosed in the periodical Siemens Components 22 (1994) number 4, pp 160-165, for measuring the shield attenuation in the certification of shielded MR systems is also constructed in the same way. The equipment required therefor is respectively made available during a maintenance or check. Leaving the high costs of the monitoring device entirely out of consideration, a continuous measurement of the RF tightness is not provided in either instance, nor can it be meaningfully achieved.
An object of the present invention is to provide a device of the type initially described, but which allows continuous and direct monitoring of the RF tightness of shielded cubicle of an MR system independently of maintenance intervals and the presence of a service technician.
This object is inventively achieved by providing an RF transmission antenna at the outside in the door region of the shielded cubicle connected to an RF output of the MR system, and a body coil of the MR system is employed as a reception antenna for determining the RF tightness of the shielded cubicle of the MR system and is connected to a monitoring receiver. The RF signal preferably is modulated in a specific manner in order to be able to filter it out of the noise better, even in the case of relatively minute leaks, and thus to allow detection of incipient RF leaks very early. The capability of modulating the RF signal is already present in conventional MR systems.
This direct utilization of the MR system and its components for the monitoring of the RF tightness of a shielded cubicle makes this monitoring very simple and inexpensive since nearly no separate assemblies at all are required for the monitoring device; rather, system parts that already exist in an MR system can be put to use for this monitoring of the RF tightness.
A signal to be radiated for this purpose is generated by the frequency processing in the MR electronics, with the signal being amplified if necessary and forwarded to an antenna structure in the proximity of the cubicle door. This antenna structure can be either a dipole antenna arranged next to the side of the door that opens or, preferably, can be a loop antenna integrated into the doorframe. The frequency of the signal preferably is at the operating frequency of the MR system so that the RF tightness is monitored in exactly that range wherein, of course, noise source, if present, could also cause the greatest harm in an MR examination.
At the same time, reception is carried out with the body coil or a local coil in the cubicle, and the signal level at the transmission frequency of the outside antenna also is acquired. A suitable point in time for the implementation of this monitoring of the RF tightness, for example, is during the adjustment for preparing for a measurement, during which time it can be assumed that the cubicle door has been properly closed. If a level of the detected signal that lies above a defined threshold is received in this measurement, the system generates a corresponding message that, for example, can result in the generation of an alarm for the operator (xe2x80x9cneed to check tightness of the RF cubicle, image quality can be negatively influencedxe2x80x9d or the like).
An advantage of the inventive method is a true acquisition of the relevant quantities (tightness of the shielded cubicle) immediately at the relevant point in time for an examination, and an advantage of the apparatus for the implementation of this method is that, except for the RF transmission antenna, practically no additional components at all are required.